1. Field of the Invention
The present invention relates generally to electrically powered cryogenic coolers [hereinafter called "cooler(s)"] which are used to cool the contents of vacuum chambers [the contents hereinafter called "object(s)"] to extremely low temperatures, say, on the order roughly of 100 degrees Kelvin or less, and, more particularly, but not by way of limitation, to a novel non-linear coupling for thermally coupling less cold to more cold elements in the cooler.
2. Background Art
Electrically powered coolers are an attractive alternative to a cooler cooled by cryogenic liquids (such as liquid nitrogen) in many applications because they do not require periodic replenishment of the coolant and because there is no evolution of gas in the cooling process.
The efficiency of these electrically powered coolers is relatively low; perhaps a few percent at best. High cooling power thus has serious implications on the size, weight, and power consumption of the cooler. For this and other reasons, the object(s) being cooled are almost always contained in a closed chamber which is evacuated to a low pressure to reduce the heat load. Such a low pressure is known as an "insulating vacuum".
The pressure in such a chamber will rise in time after it is evacuated because of outgassing of all the materials within the chamber and because of gas seepage or leaks past the seals of the chamber. The pressure can be maintained at a low level by various means including continuous or periodic pumping by one or more of various types of vacuum pumps, including mechanical pumps, diffusion pumps, ion pumps, turbo molecular pumps, or cryo pumps. Each of the aforementioned pumps is relatively large and/or expensive, however, compared to the adsorber pump that has historically been used to maintain vacuum in such vacuum chambers. The adsorber pump is simply a quantity of adsorbent, such as activated charcoal or synthetic zeolite, which adsorbs gas molecules when the adsorbent is cooled to cryogenic temperatures. The gas capacity of these adsorbers at cryogenic temperatures is quite large, so they will maintain low pressures for many years under normal conditions. However, if they are allowed to warm up, they will release significant amounts of the gas they have adsorbed, raising the pressure in the chamber to levels above the "insulating vacuum" range. This does not present a problem when cryogenic liquids are used for cooling, as the liquids provide enough cooling power to re-cool the adsorber even when the chamber pressure is high. As the adsorber is cooled, it will re-adsorb the gas and restore the "insulating vacuum" condition in the chamber.
When electrically powered coolers are used, however, they may not have enough power to overcome the heat transferred to the object(s) through the residual gas in the chamber. If the heat load of the object(s) exceeds the cooling power of the cooler, a stall condition is created. In this condition, the temperature does not get low enough for the adsorber to pump properly and the pressure remains at the higher (non-insulting vacuum) level. This stall condition can be corrected only by pumping on the chamber to reduce the pressure.
Accordingly, it is a principal object of the present invention to provide means for enabling electrically powered coolers to cool adsorbers in the presence of a high heat load associated with the higher pressure (non-insulating vacuum) of a warm system.
It is a further object of the invention to provide such means that operates automatically without manual intervention.
It is an additional object of the invention to provide such means that can be economically implemented.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.